Interpretive Summary: The frustrations associated with conserving high-quality alfalfa silage during periods of unstable or inclement weather are widely known. Within this series of experiments, rainfall events were applied to wilting alfalfa forages by both simulated (using a calibrated rainfall simulator) and natural methods across four independent forage harvests. Generally, simulated rainfall was applied to alfalfa under controlled conditions in which forages were relatively wet at the time of application, and then were dried to their final moisture endpoint under near ideal conditions within a constant temperature/humidity environmental chamber. Indicators of ensilability, as well as measures of nutritive value, changed only minimally as a result of rain damage. Consistently, there were reductions in concentrations of water-soluble carbohydrates (sugars) and starch, but changes in water-soluble carbohydrates under ideal post-wetting drying conditions were relatively modest. When forages were subjected to natural rainfall events followed by prolonged exposure under field conditions, indicators of ensilability were much less desirable, as were measures of nutritive value. Based on these experiments, ensilability of alfalfa should be affected only minimally by single rainfall events applied to relatively wet forages, as long as these events are followed by rapid dehydration to moisture concentrations suitable for making silage; however, attaining acceptable silage fermentation with forages subjected to prolonged exposure under less than ideal drying conditions is likely to be far more problematic.

Technical Abstract:
The frustrations of forage producers attempting to conserve high-quality alfalfa (Medicago sativa L.) silage during periods of unstable or inclement weather are widely known. Our objectives were: i) to assess the effects of simulated or natural rainfall on indicators of ensilability, such as pH, buffering capacity, water-soluble carbohydrates (WSC), and also starch; ii) to use these data as inputs to calculate the threshold moisture concentration that would prohibit a clostridially dominated fermentation (MAX); and iii) to further evaluate the effects of rainfall events on measures of nutritive value. Rainfall events were applied to wilting forages by both simulated and natural methods over multiple studies distributed across 4 independent forage harvests. Generally, simulated rainfall was applied to alfalfa under controlled conditions in which forages were relatively wet at the time of application, and subsequently were dried to final moisture endpoints under near ideal conditions within a constant temperature/humidity environmental chamber, thereby limiting post-wetting wilting time to = 21 h. As a result, indicators of ensilability, as well as measures of nutritive value, changed only marginally as a result of treatment. Consistently, there were reductions in concentrations of WSC and starch, but changes in WSC were relatively modest, and post-wetting concentrations of WSC may have been buoyed by hydrolysis of starch. When forages were subjected to natural rainfall events followed by prolonged exposure under field conditions, indicators of ensilability were much less desirable. In one study in which alfalfa received 49.3 mm of natural rainfall over a prolonged (8-d) field-exposure period, fresh pH increased (P < 0.001) from 6.48 to 7.43 within all forages exposed to these extended, moist wilting conditions. Furthermore, there also were sharp reductions in buffering capacity (410 vs. 337 meq/kg DM; P < 0.001), WSC (6.13 vs. 2.90%; P < 0.001), starch (2.28 vs. 0.45%; P < 0.001), and MAX (62.7 vs. 59.4%; P < 0.001). Based on these experiments, the potential for good fermentation is affected only minimally by single rainfall events applied to relatively wet forages, provided these events are followed by rapid dehydration to moisture concentrations suitable for making silage. However, attaining acceptable silage fermentations with forages subjected to prolonged exposure under poor drying conditions is likely to be far more problematic.